1. Field of the Invention
This invention relates to indirect radiant heaters, in particular, indirect flat radiant panel heaters. This invention relates to liquid and gaseous fuel flat radiant panel heaters which are self-recuperating and which produce low and ultra-low NOx emissions, typically less than about 100 vppm. This invention further relates to an indirect radiant heating apparatus employing flat radiant panel heaters.
2. Description of Related Art
Gas- and oil-fired, indirect radiant heaters are known to those skilled in the art. Typically, such radiant heaters take two forms—radiant tubes in which the heat radiating surface is a tubular structure constructed of suitable materials, and flat radiant panels. When employed in industrial heating applications, such as industrial furnaces and the like, the radiant tube heaters typically extend through the refractory roof, floor and/or sidewalls of the furnaces into the interior space. In contrast thereto, flat radiant panel heaters are a particularly attractive alternative because the compactness of such heaters allows easy installation in the refractory sidewalls, roof and, if necessary, in the furnace hearth. This allows displacement of the conventional refractory walls, refractory roofs and/or refractory hearths and provides a uniform heat flux to all sides of the load with low NOx levels in the combustion products and a high degree of temperature uniformity of the working surface, typically ±18° F. And, because they do not protrude into the interior space of the furnace, such space remains available for displacement by additional product material.
Radiant heaters may be of two designs, an open design in which the products of combustion can come into contact with the load, which, for example, in cases where the load is foodstuffs, may be undesirable, and a closed design in which the products of combustion are exhausted away from the load. It will be apparent to those skilled in the art that radiant heaters having a closed design are preferable to those having an open design. One such closed design radiant heater is taught by U.S. Pat. No. 5,483,948 to van der Veen.
The radiant heater of U.S. Pat. No. 5,483,948 comprises a closed housing which includes a combustion chamber, a flat radiant panel directed towards the article being heated, which panel forms a boundary of the combustion chamber, and at least one burner in which the combustion process is carried out in two stages, the first stage combustion being carried out external to and upstream of the combustion chamber and the second stage being carried out in the combustion chamber itself. The flame formed by the burner is directed at the radiant panel and gas guiding means are provided for recirculating the flue gases that have been passed along the radiant panel to the first or second combustion stage. It is stated that the contact of the radiant panel with the flue gases results in a cooling of the flue gases and that recirculating these cooled flue gases to the first or second combustion stage results in a cooling of the flame, which, in turn, results in a reduction in the NOx content of the flue gases. Recirculation of the cooled flue gases to the first combustion stage is effected through external recirculation means whereby the flue gases that have passed the radiant panel are mixed with the combustion air being fed to the first stage of the burner. Recirculation of the flue gases to the second stage is effected through internal recirculation in the combustion chamber with the walls of the combustion chamber serving as guiding means. It will be appreciated by those skilled in the art that the use of flue gas recirculation in the described manner adds a substantial degree of complexity to the burner design, particularly since means for recirculating the flue gases to both the first and second stages must be provided.